The demand for power varies significantly over the course of the day. There is also variation in the generation of power, with an increasing proportion of power available from renewable energies during the course of the day. In order to be able to compensate for an oversupply of power in periods with a lot of sun and strong wind when demand for power is low, controllable power plants or storage means are required to store this energy.
One of the solutions include the conversion of electrical energy to products of value which can especially serve as platform chemicals or synthesis gas which comprises carbon monoxide and hydrogen. One possible technique for conversion of electrical energy to products of value is electrolysis. The electrolysis of water to hydrogen and oxygen is a method known in the prior art. But the electrolysis of carbon dioxide to carbon monoxide has also been a subject of research for a few years, and there are efforts to develop an electrochemical system that can reduce an amount of carbon dioxide in accordance with economic interests.
At present, about 80% of global energy demand is covered by the combustion of fossil fuels, and the processes of combustion thereof cause global emissions of about 34 000 million tons of carbon dioxide into the atmosphere per year. Carbon dioxide is one of the “greenhouse gases”, the adverse effects of which on the atmosphere and the climate are a matter of discussion. Utilization of this carbon dioxide is therefore desirable.
An electrolysis unit may include a low-temperature electrolyzer in which carbon dioxide as product gas is metered into a cathode space with the aid of a gas diffusion electrode. The carbon dioxide is reduced to carbon monoxide at a cathode of the electrochemical cell, and water is oxidized to oxygen at an anode. Owing to diffusion limitations at the cathode, use of an aqueous electrolyte can result not only in the formation of carbon monoxide but also in the formation of hydrogen, since the water in the aqueous electrolyte is likewise electrolyzed. In the current state of the art, typically a maximum of 70% of the carbon dioxide used is electrochemically converted. Assuming a conversion level of carbon dioxide of 50% and noting that the Faraday efficiency for carbon monoxide and hydrogen is 50% in each case, the result is a product gas having a composition of carbon monoxide to hydrogen to carbon dioxide in a ratio of 1:1:1.